Some of the seats of a vehicle such as the front seats include, for example, a seat lifter device, which allows a seat occupant to adjust the vertical position of a seat cushion in accordance with the size of the seat occupant to obtain a desired eye level. Conventionally, various types of such seat lifter devices have been proposed. For example, a seat lifter device disclosed in Patent Document 1 includes lifter mechanisms, which are mounted on base plates on upper rails. The lifter mechanisms adjust the vertical position of the seat cushion. Brackets are fixed on the base plates, and a seat back is fixed on the brackets via reclining mechanisms. According to this seat lifter device, only the seat cushion moves vertically with respect to the base plates and the seat back.
In a seat lifter device disclosed in Patent Document 2, the seat cushion is supported on lower frames via front links and rear links that form quadric crank chains. The front links and the rear links adjust the vertical position of the seat cushion. The seat back is fixed on the seat cushion via the reclining mechanisms. According to this seat lifter device, unlike the device of Patent Document 1, the seat cushion and the seat back integrally move up and down.